A model system fortified with ascorbic acid or thiamin was equilibrated to its BET monomolecular moisture content at 20 degrees C on both the adsorption and desorption isotherms, and at a moisture content below the BET monomolecular content on the adsorption isotherm. All model systems were stored in moisture proof containers at 20 degrees C to prevent any change in water activity during storage. The rate of thiamin and ascorbic acid degradation was determined as a function of water activity. Thiamin was shown to be stable in the dehydrated model system at water activities below and at the BET monomolecular content on the adsorption and desorption isotherm. Calculation of per cent loss of thiamin showed less than 2.25% loss under these storage conditions. Ascorbic acid losses followed first order kinetics for both reduced and total ascorbic acid. The per cent loss of reduced ascorbic acid, calculated from the first order rate constants, after six months storage at 20 degrees C was approximately 98 per cent. Rate constants describing the loss of total ascorbic acid were significantly lower. This lowering was due to the slower destruction of dehydroascorbic acid under these storage conditions.